The present invention relates to nets, for example cargo nets to restrain the movement of cargo in the fuselage of aircraft.
It is well known to use nets as barriers on aircraft to restrain cargo that would otherwise move and penetrate into areas occupied by aircraft personnel, critical equipment and systems, access door or emergency access routes during the acceleration and deceleration of the aircraft during take-off and landing and also during the extreme deceleration of the aircraft during emergency landing.
Conventionally, such nets comprise a network of textile straps to which securing means are fitted, e.g. shackles, quick release fittings, snap latches, single and double stud floor fittings to connect the net to the aircraft structure.
The success of the net to act as a barrier to the moving cargo relies on the net forming a profile when loaded that does not encroach into the areas it is intended to protect, whilst safely distributing the resulting tensile load into the aircraft structure.
In conventional textile nets, the straps are stitched at their intersections which helps to maintain the shape of the net and also ensures that the strap lengths between intersections are approximately equal. This means that when the load is applied to the net the load distribution throughout the net should be as designed. However, manufacturing tolerances used in the construction of the net mean that it is impossible to make the individual lengths exactly equal. Nets made of a high elongation material are able to compensate for this. If a member of the net from one stitched intersection to the other is shorter than it should be, all that happens, is that it will experience a magnitude of load sooner than it should have, the material elongates, which causes the load to be shared to the other members in the net.
Such known nets suffer from disadvantages. The textile straps elongate under tensile load, which although in some cases can be an advantage in forming the desired profile when restraining the shifted cargo, in cases when the net is fitted in close proximity to the areas occupied by aircraft personnel, critical equipment and systems, or emergency access routes, the elongation of the net must be strictly limited. Limitation of the elongation of the net is currently achieved through the employment of multiple layers of textile straps. Multiple layers of textile straps increase the volume of the cargo net and, depending on the number of layers required, can prove very difficult to manufacture.
It would therefore be preferable to make nets out of materials having a much lower elongation, e.g. nets comprising metallic fibre cables or plastics fibre materials. Nets made of plastics fibre materials (such as aramid fibre materials, e.g. KEVLAR®) would be highly desirable since such nets would have a much lighter weight, a much higher strength to weight ratio and prevent cargo from penetrating into unwanted areas.
A net constructed of low elongation material in the conventional manner with stitched intersections would experience a number of problems. For a low elongation material net to balance out as described before, the accuracy of the net manufacturer would need to be much higher. If a member of the net from one stitched intersection to the other was shorter than it should be then it would experience a magnitude of load sooner than it should have but, because of the low elongation properties of the material it would not elongate sufficiently to share the load to the other members of the net and consequently it would experience a larger magnitude of load for a longer period of time thus making that member of the net more susceptible to breakage.
The manufacturing tolerances used for both high and low elongation material are identical and it is not practical to have increased accuracy for low elongation materials and hence not possible to compensate for low elongation material in the manner described for high elongation materials.